As a general rule, foams are produced by an extrusion operation in which a foamable composition is extruded into a shaped article and the shaped article allowed to expand and foam; or by a compression molding operation. Producing a foam by a compression molding operation is a relatively simple operation which involves, initially, placing a preform of a foamable composition, having a volume equal to about 103 percent of the volume of the mold cavity to provide a seal around the cavity, into the cavity of the mold, placing the filled mold in a hydraulic press and continuing according to Procedures 1 or 2 described below. The foamable composition itself contains a polymer, an organic peroxide, a gas generating compound and generally, well known additives such as anti-oxidants, lubricants and the like.
Procedure 1
The temperature of the hydraulic press is maintained at high levels, generally in excess of about 160.degree. C. and the residence time in the press is such as to achieve substantially full activation of the peroxide and the gas generating compound. The hydraulic press is then opened, allowing the composition to expand and foam.
Procedure 2
The temperature of the hydraulic press is maintained at a level such that the composition is partially cured. The partially cured composition, is preferably cooled, removed from the compression mold, placed in a hot oven and allowed to expand and foam.
Extruding foamable compositions into shaped articles and allowing the shaped articles to expand and foam involves a different set of conditions than those set forth in Procedures 1 and 2. In the production of foams by the "extrusion" technique, the rate of crosslinking of the polymer, i.e. peroxide cure, and the rate of decomposition of the gas generating compound must be carefully balanced. If the rate of crosslinking is too rapid, expansion of the foamable composition will be unduly restricted. On the other hand, if the rate at which the gas generating compound decomposes substantially exceeds the rate at which crosslinking occurs, the evolved gases will escape and little, if any, expansion of the composition will take place.